Netiquette, is the term given to given to the correct way of using the Internet. Internet etiquette. Netiquette exists for a reason, namely to make the net easier and faster to use for everyone. Whilst opinions may vary on what is the correct way of using any aspect of the net, and minor transgressions will be ignored, major transgressions such as sending Spam email or news posts can result in your Internet service provider terminating your account. Netiquette is no different to everyday life, what sort of reaction would you get if you walked into a library with a portable Hi-Fi turned up to full volume, you'd asked to turn it off, and when you didn't you'd be kicked out. The same applies on the Internet if you annoy people with bad netiquette, people will point out your mistake to you, so it's best to avoid making the mistake in the first place. For any aspect of the Internet there will be help file FAQ Frequently Asked Questions - read them.
Wasting bandwidth - Bandwidth is the maximum amount of data a network or part of a network can carry, all networks have a finite bandwidth and as it is used up the network gets slower for everyone.
Not reading the FAQ - When someone has gone to the trouble of creating a FAQ guide for new users, not reading it and then asking the most frequently asked question can really annoy people especially if your the hundredth newbie to do ask it that month. This is mainly applicable to Usenet groups.
Spamming - Whilst it is really covered under wasting bandwidth,Spamming is the cardinal offence on the net, almost everyone hates spammers, if you send spam expect to lose you net access.